1. Field of the Invention
This invention relates to a clock signal extraction system, and more particularly to a phase locked loop (PLL) circuit which extracts a clock signal from a readout signal in an apparatus for recording and reproducing high density information onto and from a recording medium in the form of a disk.
2. Description of the Related Art
In recent years, with the improvement in performance of computers, large capacity file apparatuses such as hard disk drives (HDD) and CD-ROM (compact disk read only memory) drives have exhibited a remarkable spread.
When information digitally recorded in a high density format on a recording medium is reproduced, a synchronizing clock signal is extracted from a reproduced analog signal and the digital information is detected using the clock signal. Detected information, particularly from a data filing system, is required to have a high degree of reliability, i.e., having an accuracy represented by a bit error rate of lower than 10.sup.-5 with regard to an optical disk and lower than 10.sup.-9 with regard to a magnetic disk. Accordingly, the clock signal must be extracted following up irregular rotation of a spindle accurately. Where the clock signal includes jittering, the reliability of the disk system deteriorates significantly.
FIG. 13 shows a conventional PLL circuit. Referring to FIG. 13, a phase comparator 22 compares the phases of an input signal REF to the PLL circuit with an output clock signal PCLK of the PLL circuit and outputs a phase error signal. High frequency components of the phase error signal are suppressed by a loop filter 6, and an output signal of the loop filter 6 is supplied as a control voltage to a voltage controlled oscillator (VCO) 7.
The VCO 7 outputs a clock signal PCLK of a frequency corresponding to the output voltage of the loop filter 6. The clock signal PCLK is fed back to the phase comparator 22, thereby forming a closed loop for automatically controlling the frequency and the phase difference.
In order to cope with multi-media requirements, further reduction in size and increase in density Is required for present day data filing systems. Given an optical disk onto and from which information is recorded and reproduced using a beam spot having a diameter which increases in proportion to the wavelength of the laser and in inverse proportion to the numerical aperture (NA) of the objective lens, it is difficult to achieve an increase in recording density by simply reducing the beam diameter because of problems with thermal noise and/or tilting.
In order to realize high density recording with a hard disk, the head must be small and a head floating amount (on the order of sub-microns) must be realized. Therefore it is difficult to raise the signal quality.
Recently, a partial response maximum likelihood (PRML) signal processing technique (in which a partial response method is used for equalization to shape a readout waveform and viterbi detection, which is a maximum likelihood detection method or the like, is used for detection of data) has begun to be utilized. With the PRML signal processing technique, information can be reproduced well even from, for example, a readout waveform having an eye pattern which is in an almost collapsed condition. This is accomplished by applying viterbi detection, which has conventionally been used in the field of communications, to a disk system.
However, when data are reproduced from a recording medium recorded in a high density, a relatively low resolution results in low energy of a clock frequency (clock signal) included readout signal. Consequently, the signal to noise ratio (SNR) of the input signal to the PLL circuit is decreased. Consequently, sampling jitters are increased and the reliability of detected information is deteriorated.
Where the PRML signal processing technique is employed, indeed it is theoretically possible to detect data even from a waveform which has an insufficient resolution, but where excessive jittering occurs in a reproduced clock signal, the performance of the PRML signal processing technique cannot be extracted fully.
Accordingly, it is a significant subject of high density recording to extract a clock signal accurately from a waveform having an insufficient resolution.